See README file (does not exist in pre-7.9 tree) in 7.9 source tree trace driver directory for instructions. Basically:
1) set the name of the trace output file in GALLIUM_TRACE environment var
2) call trace_screen_create() passing it the screen ptr created for the actual device
3) output file is in XML format and needs the formatting style sheet. Put trace.xsl in the same directory as outputfile and open output file with IE or Firefox.
Why isn’t this kind of information made more accessible from DRI wiki or mesa3d portal?
Trace driver background:
http://jrfonseca.blogspot.com/2008/09/tracing-gallium3d.html
Ed's Gallium3D Survival Log 